There are radiated emission regulatory requirements that must be satisfied by any commercial product. One of these requirements is called the electromagnetic compatibility (or EMC) requirement, which requires that products must not radiate excessive electromagnetic radiation into their intended environment. For electronic products having computational devices therein, electromagnetic radiation may be difficult to restrict from the product's intended environment since there are typically ventilation openings or ducts for air circulation in order to dissipate heat. In particular, such computational devices may generate high frequency electromagnetic radiation, which is characterized by short wavelengths, wherein such radiation is easier to leak through, e.g., ventilation openings.
One source of high frequency electromagnetic radiation generated by computational devices is the voltage oscillations along conductive traces within such devices. In particular, the voltage changes associated with the rise and fall times of the waveforms of bits transmitted along such traces can radiate high frequency electromagnetic radiation. For example, it is known that decreases in the rise and/or fall times increases high frequency energy being radiated. Assuming the rise and fall times are approximately the same, the highest frequency of the radiated energy, in some circumstances, can be roughly characterized as one over twice the rise time. Accordingly, very short rise and/or fall times (e.g., on the order of about 200 picoseconds or less) for digital voltage waveforms can result in unacceptably high frequency energy being radiated from a computational device. However, as computational devices become increasingly faster, bit periods tend to decrease, and accordingly, the corresponding rise and fall times within such bit periods tend to decrease thereby generating increasingly more radiated high frequency electromagnetic energy. Thus, as the computational processing speed increases, additional measures must be taken to make sure that an undesirable amount of high frequency radiation is not released into the environment where it may harm people and/or affect other devices.
One way to reduce the release of such high frequency radiation can be to reduce the number or size of the apertures through which such radiation may exit a housing for a computational device and thereby enter the environment. However, as mentioned above, such a technique could require sophisticated ducting, more powerful ventilation fans, more electromagnetic shielding in the housing, and/or greater attention during manufacturing to properly seal small unintended openings where such radiation could exit.
An alternative approach to reduce the release of such high frequency radiation is to lengthen the rise and fall times of the digital signals by using a signal filtering mechanism. The most straightforward mechanism is a low pass filter that increases the rise and/or fall times. However, conventional techniques for implementing such low pass filters do not perform well when the rise and fall times are very small (e.g., on the order of about 200 picoseconds or less). For example, commercially available discrete resistors, discrete capacitors, discrete conductors operate substantially differently when exposed to such rapid voltage changes of very small rise and/or fall times. In particular, parasitic effects are generated by such components so that a presumed low pass filter circuit having such discrete components will not properly lengthen the rise and/or fall times. For example, a commercial capacitor will generally only behave as a capacitor up to a frequency range of about 25 to 30 megahertz. Beyond this frequency range, such a capacitor will behave as an inductor.
Thus, it would be desirable to be able to effectively attenuate the release of high frequency electromagnetic radiation in a high-speed computational device in a straightforward manner, and without instituting elaborate measures for trapping such high frequency electromagnetic radiation within the confines of a housing for the device.